Greg Smith was one of the luckiest ducks on Wall Street last March before he quit his job at Goldman Sachs and lambasted the firm in an op-ed piece in the New York Times.

He was living a life most people only dream of -- $150-per-person dinners, $500-a-hand poker bets and a $2,500-a-month Manhattan apartment.

A pharmacist's son from South Africa, Smith had parlayed a full scholarship to Stanford into an internship and then a job at Goldman Sachs.

By age 32, he was ensconced in the bank's London office, making $700,000 a year as head of its U.S. equity derivatives business for Europe, Africa and the Middle East. Until he quit.

Why I Left Goldman Sachs: A Wall Street Story is an expanded version of Smith's op-ed. Anyone who expected an expose will be disappointed, but Smith has written a field guide to the culture of Goldman Sachs and a fly-on-the-wall account of Wall Street on the skids.

Smith says he was drawn to Goldman Sachs by its mantra of "always put the clients' interests first." Its storied history of interns past who went on to become senators, treasury secretaries and central bank governors didn't hurt either.

Smith was impressed that the higher-ups sidelined interns who fudged answers. Interns dealing with supervisors, just like traders dealing with clients, were expected to be truthful. If they didn't know an answer, they were expected to say, "I don't know, but I'll find out."

Smith says he "drank the Kool-Aid" at this bank that took its culture and its fiduciary responsibility to clients very seriously.

After CEO Hank Paulson left to become Secretary of the Treasury and Smith started seeing things that didn't fit the corporate culture, he didn't blow any whistles immediately.

It took almost five years for Smith to come to the conclusion that venerable Goldman Sachs had morphed into a firm more concerned with helping itself than helping its clients.

Goldman Sachs CEO Lloyd Blankfein responded to the column with a letter to employees that made Smith look like the cheese who stands alone. The letter said the company's internal polls show 89% of employees believe the bank provides exceptional service to clients.

Smith had a front-row seat as the bank reeled along with the world financial markets. Literally a front-row seat in one case: Warren Buffet stood next to his desk the day he announced his $5 billion investment in Goldman Sachs.

He said Goldman's culture slowly segued until partners only wanted business that could make the firm a lot of money. It was, Smith said paraphrasing rap producer Sean Combs, all about the Benjamins.

The book is dotted with good observations: Smith describes the "partner laugh," a heartier-than-usual guffaw you'd hear around you whenever a trader was trying to impress a higher-up. And facts most readers don't know: Hank Paulson is an ardent environmentalist and an avid bird watcher.

And it makes some news items crystal clear: Although it seemed Paulson made a tremendous financial sacrifice when he left his job at Goldman and sold all his stock to serve as Treasury secretary, he was actually lucky to sell at the top of the market and, because, he had to sell to accept the government post, he avoided paying capital gains on $500 million.

Smith clearly explains financial terms such as structured derivatives -- those complicated financial products whose returns are often linked to interest rates, commodities or indices. He writes that buying a structured derivative is like going into a store and buying a can of tuna with "Bumble Bee Tuna" written on the front with the bee logo, but, when you go home and open it, you find dog food inside. You think, "How can this be? They told me it was tuna." But, when you look at the very small print on the back of the can, it says, "Contents may not be tuna. May contain dog food."

Smith's insider snippets offer a peek at how at least part of Wall Street works. He writes:

• On the same day when one trader asked his wife to move their own money around to several banks to keep each deposit under the $250,000 FDIC insurance limits, he was advising a client to invest.

• Traders in the London office called the clients "muppets," which they used as a synonym for idiots.

• Traders were encouraged to go on "elephant hunts" to bag investors whose purchases or sales would garner $1 million or more in fees to the bank.

• The clients who fared worst were the clients who didn't know how to ask questions. One was overcharged by $1.5 million.

• With century-old institutions like Lehman Bros. disappearing before their eyes, panicking investors tried to get out of Goldman funding trades – agreements they made to commit cash to the firm for one year. The bank enforced stringent clauses that prevented investors from breaking the agreements.

Smith believed that went against the bank's written principle: "Our assets are our people, capital and reputation. If any of these is ever diminished, the last is the most difficult to restore." Despite this, he says, when salesmen asked a Goldman partner to loosen the clauses for clients, he answered, "If I have to choose between my reputation and my P&L, I choose my profit and loss because I can ultimately recover my reputation, but, if I lose a lot of money, I can't recover that."